1. Field of the Invention
The present invention relates to an apparatus and method for probing integrated circuits using laser illumination.
2. Description of the Related Art
Probing systems have been used in the art for testing and debugging integrated circuit (IC) designs and layouts. Various laser-based systems for probing IC's are known in the prior art. While some description of the prior art is provided herein, the reader is encouraged to also review U.S. Pat. Nos. 5,208,648, 5,220,403 and 5,940,545, which are incorporated herein by reference in their entirety. Additional related information can be found in Soref, R. A. and B. R. Bennett, Electrooptical Effects in Silicon. IEEE Journal of Quantum Electronics, 1987. QE-23(1): p. 123-9; Kasapi, S., et al., Laser Beam Backside Probing of CMOS Integrated Circuits. Microelectronics Reliability, 1999. 39: p. 957; Wilsher, K., et al. Integrated Circuit Waveform Probing Using Optical Phase Shift Detection, in International Symposium for Testing and Failure Analysis (ISTFA), 2000, p 479-85; Heinrich, H. K., Picosecond Noninvasive Optical Detection of Internal Electrical Signals in Flip-Chip-Mounted Silicon Integrated Circuits. IBM Journal of Research and Development, 1990. 34(2/3): p. 162-72; Heinrich, H. K., D. M. Bloom, and B. R. Hemenway, Noninvasive sheet charge density probe for integrated silicon devices. Applied Physics Letters, 1986. 48(16): p. 1066-1068; Heinrich, H. K., D. M. Bloom, and B. R. Hemenway, Erratum to Noninvasive sheet charge density probe for integrated silicon devices. Applied Physics Letters, 1986. 48(26): p. 1811.; Heinrich, H. K., et al., Measurement of real-time digital signals in a silicon bipolar junction transistor using a noninvasive optical probe. IEEE Electron Device Letters, 1986. 22(12): p. 650-652; Hemenway, B. R., et al., Optical detection of charge modulation in silicon integrated circuits using a multimode laser-diode probe. IEEE Electron Device Letters, 1987. 8(8): p. 344-346; A. Black, C. Courville, G Schultheis, H. Heinrich, Optical Sampling of GHz Charge Density Modulation in Silicon Bipolar Junction Transistors Electronics Letters, 1987, Vol. 23, No. 15, p. 783-784, which are incorporated herein by reference in their entirety.
As is known, during debug and testing of an IC, a commercially available Automated Testing Equipment, also known as Automated Testing and Evaluation, (ATE) is used to generate test patterns (also referred to as vectors) to be applied to the device under test (DUT). When a laser-based probing system is used for the testing, the DUT is illuminated by the laser and the light reflected from the DUT is collected by the probing system. As the laser beam strikes the DUT, it is modulated by the response of various elements of the DUT to the vectors. This has been ascribed to the electrical modulation of the free carrier density and the resultant perturbation of the index of refraction of the material. Accordingly, analysis of the reflected light provides information about the operation of various devices on the DUT.
FIG. 1 is a general schematic depicting major components of a laser-based voltage probe system architecture, 100, according to the prior art. In FIG. 1, dashed arrows represent optical path, while solid arrows represent electronic signal path. The optical paths are generally made using fiber optic cables. Probe system 100 comprises a dual laser source, DLS 110, an optical bench 112, and data acquisition and analysis 114. The optical bench 112 includes provisions for mounting the DUT 160. A conventional ATE 140 provides stimulus signals 142 to the DUT 160, and trigger and clock signals, 144, to the probe controller 170 via time-base board 155. The time-Base Board 155 synchronizes the signal acquisition with the DUT stimulus and the laser pulses.
The various elements of probe system 100 will now be described in more detail. Since temporal resolution is of high importance in testing DUT's, the embodiment of FIG. 1 utilizes prior art pulsed lasers, wherein the pulse width determines the temporal resolution of the system. Dual laser source 110 consists of two lasers: a pulsed mode-locked laser, MLL 104, source that is used to generate 10-35 ps wide pulses, and a continuous-wave laser source, CWL 106, that is pulsed electronically to generate approximately 1 μs wide pulses. The MLL 104 source runs at a fixed frequency and must be synchronized with the stimulus 142 provided to the DUT 160, via a phase-locked loop (PLL) on the time-base board 155. The output of the DLS 110 is transmitted to the optical bench 112 using fiber optics cable 115. The light beam is then manipulated by beam optics 125, which directs the light beam to illuminate selected parts of the DUT 160. The beam optics 125 consists a Laser Scanning Microscope (LSM 130) and beam manipulation optics (BMO 135). The specific elements that are conventional to such an optics setup, such as objective lens, etc., are not shown. Generally, BMO 135 consists of optical elements necessary to manipulate the beam to the required shape, focus, polarization, etc., while the LSM 130 consists of elements necessary for scanning the beam over a specified area of the DUT. In addition to scanning the beam, the LSM 130 has vector-pointing mode to direct the laser beams to anywhere within the field-of-view of the LSM and Objective Lens. The X-Y-Z stage 120 moves the beam optics 125 relative to the stationary DUT 160. Using the stage 120 and the vector-pointing mode of the LSM 130, any point of interest on the DUT 160 may be illuminated and probed.
For probing the DUT 160, the ATE 140 sends stimulus signals 142 to the DUT, in synchronization with the phase-locked loop on the time-base board 155. In synchronization with the stimulus, the MLL 104 emits laser pulses that illuminate the particular device on the DUT that is being stimulated. The light is then reflected from the DUT, but the reflection changes character depending on the reaction of the device to the stimulus signal. The reflected light is then collected by the beam optics 125 and is transmitted to two photodetectors 136, 138 via fiber optic cables 132, 134. The output signal of the photodetectors 132, 134 is sent to signal acquisition board 150, which, in turn, sends the signal to the controller 170. Using the photodetectors signals and the synch signal from the time base board 155, the controller 170 can analyze the temporal response of the DUT. The temporal resolution of the analysis is dependent upon the width of the MLL pulse.
While the arrangement depicted in FIG. 1 has been used successfully in the art, the system has several drawbacks. From the usability and performance perspective, the phase-lock requirement puts restrictions on the usable DUT stimulus trigger and clock periods. This is a burden to the user because it prevents the user from testing at arbitrary clock frequencies and it requires additional setup by the user. Additionally, noise rejection performance is dependent upon the signal-processing algorithm. Optimizing this algorithm is difficult. From the economic perspective, the MLL is expensive, and custom electronics and software must be developed. The CWL source is also custom built to give the best vibration noise rejection, and its output must match the MLL in wavelength and spectral width. All these custom elements increase the complexity and the overall cost of the system.
A major difficulty encountered by all laser-base probe systems is deciphering the weak modulation in the reflected signal, which is caused by the response of the DUT to the stimulus. Another difficulty is noise introduced into the signal by the DUT's vibrations. Various beam manipulation optic, 135, designs have been used in the art in an attempt to solve these difficulties. FIG. 2 is a diagram illustrating standard amplitude detection mode used in the prior art. In FIG. 2, a laser probe is used to probe specific device 210, such as a transistor's source or drain of a DUT. A beam splitter 220 is used to separate the reflected beam from the incident laser beam. Amplitude modulation due to DUT interaction with the laser beam can be detected directly using a photodetector. However, the DUT interaction with the laser beam may cause changes mostly in the phase of the reflected laser beam, not its amplitude. Consequently, the signal strength would be too weak for pure amplitude detection. Additionally, DUT vibrations cause amplitude variations that are much stronger than the variation from the DUT activity of interest. This necessitates noise rejection schemes to make such an arrangement practical.
Since the DUT interaction with the laser beam causes change in the phase of the reflected beam, various phase detection schemes have been developed for the beam manipulation optics 135. FIG. 3 is a diagram illustrating phase detection scheme with Michelson Interferometer arrangement to convert phase to amplitude. This scheme is also referred to as Phase-Interferometric Detection, or PID, mode. To detect phase modulations, a portion of the incident beam from the laser source is directed into a reference arm consisting of a lens 340 and a mirror 330, using beam splitter 320. The remaining portion of the incident beam is directed to a specific area of interest on the DUT, and upon reflection it is modulated according to the DUT's response to a stimulus signal. The light beam, 355, reflected by the DUT, and the light beam, 335, reflected by the reference arm mirror 330, are then spatially recombined into a single beam 365 so that they can interfere. The interference effect converts relative phase differences between the reflected beam 355 and the reference arm beam 335 into amplitude differences in combined beam 365, which can then be detected by a photodetector.
While this arrangement helps detect phase variations caused by the DUT, using this optical arrangement exposes the system to additional noise source from phase variations caused by DUT vibrations. The DUT vibrations still modulate reflected DUT beam amplitude, but now also modulate the DUT beam phase, which generates larger resultant reflected beam intensity modulations. Additional adjustments that are required in order to get best performance include reference arm power control and reference arm mirror position control (to set phase offset between DUT and Reference Arm powers). Also, alignment of reflected DUT and reference arm beams can be difficult.
FIG. 4 is a diagram illustrating another scheme, generally known as (spatial) differential probing (SDP) for phase detection. A Wollaston prism, 430 is used to generate the two spatially separated beams, 422, 424. The two beams, 422 and 424, have orthogonal, linear polarization states. One beam, e.g., 422, is directed to the DUT active region; while the other beam, say, 424, can be directed to either an inactive region, or to an active region with complementary modulation. The advantage of the latter option is that the measured signal modulation is increased because the relative phase modulation between the two beams is doubled. In the particular example of FIG. 4, the two beams are directed to the drains of the p- and n-FETs of an inverter, which generates complementary modulations of the beam. Using this scheme the phase noise is reduced relative to the scheme illustrated in FIG. 3, because each beam is directed at the DUT, so that the DUT vibrations will tend to modulate the phase of both beams similarly.
As can be understood, various IC's have different layouts and different devices on the IC's have different dimensions. Therefore, using this embodiment for each new IC the user needs to decide where to place each beam for each test and each device to be tested within the chip. Moreover, since the beam needs to be placed at various locations on the chip, the system needs to be designed so that the beam separation is adjustable, which complicates the optics design. Additionally, the intensity ratio of the beams must be variable since the reflectivity of the regions where they are placed can differ. Power matching between the two beams is required for best results.
Experience with devices as depicted in FIG. 4 has shown that DUT vibrations can still generate amplitude fluctuations, even when a differential detection mode is used. Differential detection isn't completely effective at eliminating vibrations because it is difficult to maintain proper balance of the two spatially separated detection arms. In addition, the two return beams are not modulated identically by vibrations because they are not parked on identical structures.
FIG. 5 is a diagram illustrating time differential probing (TDP) scheme for phase detection. Two pulsed beams, 522 and 524, are time shifted by a small amount (approx. 10-100 ps) with respect to each other before DUT interaction by passing one through an optical delay line 526 (such as a thick piece of glass). After DUT reflection, the opposite beam is passed through the optical delay line to remove the relative delay between the two beams. The two beams are then allowed to interfere, 530. Since the two beams traverse a common path, DUT vibrations largely modulate both identically, making this scheme inherently vibration insensitive. However, the optics required to generate the delay are relatively complicated. Additionally, the beams are not completely identical. They have orthogonal polarization states so DUT interactions are not truly identical (birefringence effects can cause non-common mode variations of the beams). In this scheme, the resulting ‘Waveforms’ are derivatives of the logic signal and consist of positive and negative going peaks. This arrangement also requires high temporal resolution of the sampling, otherwise no signal will result. This can limit the maximum time span of a sampling window that can be used.
Accordingly, there is a need in the art for a system that will allow improved laser probing of a DUT, while simplifying operation and minimizing the system's complexity and cost.